Structural and Excitation-Dependent Photoluminescence Properties of Bi0.95-xGdxEu0.05PO4 (0 ≤ x ≤ 0.95) Solid Solutions and Their Anticounterfeiting Applications

During the past few decades, lanthanide-doped luminescent nanomaterials have been widely applied in many fields, such as display devices, white light-emitting diodes (WLEDs), lasers, data storage, development of latent fingerprints, and anticounterfeiting. Herein, we report the synthesis of Bi0.95-xGdxEu0.05PO4 solid solutions through a co-precipitation method and their potential for anticounterfeit tags. The prepared samples show excellent photoluminescence properties and emit orange-red, yellowish-green, and fuchsia colors under 254, 365, and 394 nm UV illumination, respectively. Furthermore, the replacement of Bi3+ ions with Gd3+ in Bi0.95Eu0.05PO4 enhances the luminescent intensity due to energy transfer from Gd3+ to Eu3+ ions. Optimum luminescence properties are observed in the Bi0.50Gd0.45Eu0.05PO4 sample, and it is used for anticounterfeiting applications on various surfaces. The security patterns are printed on black paper and aluminum foil using luminescent ink containing Bi0.50Gd0.45Eu0.05PO4 nanomaterials. In addition, anticounterfeiting letters are directly written on different colored surfaces such as wood, black plastic, white plastic, and a colored plastic card. These security patterns/letters exhibit different colors under different UV irradiation wavelengths and are clearly visible with high contrast and sensitivity. These patterns are stable under various humidity and temperature conditions. The intensity of these patterns/letters remains unchanged under these conditions. From systematic investigations of the optical properties and stabilities, it has been proposed that the Bi0.50Gd0.45Eu0.05PO4 nanomaterials have the potential for unclonable anticounterfeiting tags to protect valuable documents/goods from counterfeiters.